1. Field of the Invention
The present invention relates to a method for manufacturing a novel solar battery.
2. Description of the Related Art
Conventionally, as methods for manufacturing amorphous silicon films and polycrystalline silicon films used for manufacturing solar batteries, methods, such as thermal chemical vapor deposition (CVD), plasma CVD, and photo CVD, have been employed using a monosilane or a disilane gas, and generally, thermal CVD (refer to J. Vac. Sci. Technology., vol. 14, p. 1,082, 1977) and plasma CVD (refer to Solid State Com., vol. 17, p. 1193, 1975) are widely used for polycrystalline silicon and amorphous silicon, respectively.
However, in the formation of silicon films by CVD methods, there are problems in that, for example, production yield is low due to contamination of apparatuses or generation of foreign materials caused by silicon particles formed in a gas phase since gas phase reactions occur in CVD processes, a film having uniform thickness is difficult to obtain on a surface having concavo-convex areas since starting materials are gases, productivity is low since the growth rates of films are low, and complicated and expensive high-frequency generators and vacuum apparatuses are required for plasma CVD. Accordingly, further improvements of formation of silicon films have been strongly desired.
In addition, handling of starting materials is difficult since not only silicon hydrides in gas forms having strong toxicity and strong reactivity are used for CVD, but also a sealed vacuum apparatus is required since starting materials are gases. In general, the apparatus mentioned above is large and is not only expensive itself, but also large amounts of energy are consumed by the vacuum system and/or plasma generation system equipped with the apparatus, resulting in increase in production cost.
Recently, methods are proposed in which silicon hydrides in liquid forms are coated without using a vacuum system. In Japanese Unexamined Patent Application Publication No. 1-29661, a method for forming a silicon-based thin film is disclosed, in which a starting material in a gas form is liquified and adsorbed on a cooled substrate, and chemically reactive atomic hydrogen is reacted therewith so as to form a silicon-based thin film. However, there are problems in that a complicated apparatus is not only required which sequentially performs vaporization and cooling of a silicon hydride used as a starting material, but also, control of the film thickness is difficult.
In addition, in Japanese Unexamined Patent Application Publication No. 7-267621, a method is disclosed, in which a low molecular weight silicon hydride in a liquid form is coated on a substrate. However, the system of the method is unstable, and in addition, since the material is in a liquid form, a uniform coating thickness is difficult to obtain when the method is applied to a large substrate.
In British Patent No. GB-2077710A, an example of a solid polymer composed of silicon hydrides is disclosed. However, since the solid polymer mentioned above is insoluble in a solvent, a film cannot be formed therefrom by coating.
In addition, when the silicon semiconductor film described above is used for solar battery, in general, the silicon semiconductor film must be used as a p-type or an n-type semiconductor doped with atoms in Group III or Group V of the periodic table. Since the doping mentioned above is conventionally performed by thermal diffusion or ion implantation after a silicon film is formed, it must be performed in vacuum, process control thereof is complicated, and in particular, a doped silicon film is difficult to form uniformly on a large substrate.
In contrast, Japanese Unexamined Patent Application Publication No. 9-237927 discloses a method in which a solution containing a polysilane is coated on a substrate and is subsequently pyrolyzed so that silicon atoms are isolated. In the same publication as described above, as a method for forming an n-type or a p-type silicon thin film which is necessary for solar batteries, a method in which a coating film formed of a polysilane solution containing an alkyl compound, which imparts p-type or n-type conductivity, is pyrolyzed and a method in which a coating film formed from a polysilane solution is pyrolyzed in an environment containing a dopant source are disclosed. However, in the former, a uniformly doped silicon film may not be formed in some cases due to the difference in solubility of the polysilane and the alkyl compound containing a dopant, or since large amounts of carbon atoms remain in the silicon film finally formed due to carbon atoms contained in the alkyl compound, electric properties of the silicon film are degraded. In addition, in the latter, there is a problem in that a dopant amount is difficult to control.
Solar batteries, in which silicon thin films, which are necessary for solar battery manufacturing, are formed by coating using coating compositions containing silane compounds followed by heat, light, or laser treatment so that large solar batteries are easily manufactured at low cost. In addition, another object of the present invention is to provide a totally novel method for manufacturing solar batteries, in which thin films used for solar batteries other than silicon films are also formed by coating liquid materials followed by heating.
Through intensive research by the inventors of the present invention to accomplish the objects described above, the inventors of the present invention discovered a method in which large solar batteries could be manufactured at lower cost, and as a result, the present invention was completed. That is, according to the present invention, a method for manufacturing a solar battery having a structure in which at least two semiconductor thin films are disposed one over the other between a pair of electrodes, each semiconductor thin film differing from the other in the impurity concentration thereof and/or the type of semiconductor, may consist of a step of coating a liquid coating composition containing a silicon compound so as to form a coating film and a subsequent step of converting the coating film into a silicon film by heat treatment and/or light treatment.
In the method for manufacturing a solar battery of the present invention, which has a structure in which at least two semiconductor thin films are disposed one over the other between a pair of electrodes, each semiconductor thin film differing from the other in the impurity concentration thereof and/or the type of semiconductor, the formation of at least one of the semiconductor thin films may consist of, a step of coating a liquid coating composition containing a silicon compound so as to form a coating film and a subsequent step of converting the coating film into a silicon film by heat treatment and/or light treatment.
In a preferable embodiment of the method for manufacturing a solar battery, according to the present invention, as the liquid coating composition described above, a coating composition is used which may consist of, a solvent and a cyclic silicon compound (in particular, a silane compound) represented by the formula SinXm(X stands for a hydrogen atom and/or a halogen atom, the subscript n stands for an integer of 5 or more, and the subscript m stands for an integer of one of n, 2nxe2x88x922, and 2n), or which may consist of a solvent and a silicon compound (in particular, a modified silane compound) represented by the formula SiaXbYc (X stands for a hydrogen atom and/or a halogen atom, Y stands for a boron atom or a phosphorus atom, the subscript a stands for an integer of 3 or more, the subscript c stands for an integer of 1 to a, and the subscript b stands for an integer of a to 2a+c+2). In particular, in the structure of a solar battery, a coating composition comprising the cyclic silicon compound (in particular, a silane compound) represented by the formula SinXm described above is used to form an i-type conductive silicon film, and a coating composition which may consist of a silicon compound (in particular, a modified silane compound) represented by the formula SiaXbYc described above is used to form a p-type or an n-type conductive silicon film.
As the coating composition described above, a liquid composition may be used which may consist of a solvent and a mixture at an optional mixing ratio of a silicon compound (a non-modified silane compound) represented by the formula SinXm described above and a silicon compound (in particular, a modified silane compound) represented by the formula SiaXbYc described above. Accordingly, a p-type or an n-type silicon film can be obtained, in which the carrier concentration is controlled.
The step of converting the coating film into a silicon film by heat treatment and/or light treatment, described above, may further may consist of a removing step of removing a solvent in the coating film by heat treatment and a converting step of converting the coating film into an amorphous silicon film by heat treatment at a higher temperature than that in the removing step and/or light treatment.
The amorphous silicon film described above may be converted into a polycrystalline silicon film by laser annealing and/or lamp annealing.
The solvent for the coating composition described above is preferably a hydrocarbon solvent having a vapor pressure of 0.001 to 200 mm Hg at room temperature in view of handling.
The coating composition may be coated by using an inkjet printer head. In the case mentioned above, the viscosity thereof is preferably 1 to 50 mPaxc2x7s, and the surface tension thereof is preferably 20 to 70 dyn/cm.
In the silicon compound (in particular, a silane compound) represented by the formula SinXm (X stands for a hydrogen atom and/or a halogen atom, the subscript n stands for an integer of 5 or more, and the subscript m stands for an integer of n, 2nxe2x88x922, or 2n), the subscript n is preferably 5 to 20. In addition, in view of handling, the silicon compound described above is preferably a compound in which the subscript n is 5 or 6 or is preferably a mixture thereof.
In the silicon compound (in particular, a modified silane compound) represented by the formula SiaXbYc (X stands for a hydrogen atom and/or a halogen atom, Y stands for a boron atom or a phosphorus atom, the subscript a stands for an integer of 3 or more, the subscript c stands for an integer of 1 to a, and the subscript b stands for an integer of a to 2a+c+2) is preferably a compound in which the sum of a and c is approximately 5 to 20 or is preferably a mixture thereof. In particular, in view of handling, the silicon compound described above is preferably a compound in which the sum of a and c is 5 or 6 or is preferably a mixture thereof.
In a preferable embodiment of the method for manufacturing a solar battery of the present invention, the formation of electrodes and conductive films used for wiring connected to the electrodes, may consist of a step of forming a coating film on a substrate by coating a liquid material containing a metal, a step of heating the coating film so that the coating film is converted into a metal film, and a step of patterning the coating film and/or the metal film. The formation of the electrodes and the conductive films described above may be performed by, for example, a step of forming a metal film on a substrate by plating, and a step of patterning the metal film. The formation of the electrodes and the conductive films described above may be performed by, for example, a step of forming a coating film on a substrate by coating a liquid material composed of an organic compound containing indium and tin, a step of heating the coating film so that the coating film is converted into an indium-tinoxide (ITO) film, and a step of patterning the coating film and/or the ITO film.
When the wiring is formed connected to the electrode, when necessary, insulating films are formed. The formation of the insulating films may be performed by a step of forming a coating film on a substrate by coating a liquid material containing a polysilazane, a step of heating the coating film so that the coating film composed of a polysilazane is converted into a SiO2 film, and a step of patterning the coating film and/or the SiO2 film.
In a preferable embodiment of the method for forming a solar battery of the present invention, it is particularly preferable that substantially all processes for forming films including silicon films, electrodes, conductive films, and insulating films be performed by using starting materials composed of liquid materials, and vapor phase depositions and vacuum processes be not employed. According to the method described above, large scale and expensive vacuum processes are omitted, and solar batteries can be manufactured at lower cost compared to that of conventional ones.
In the step of coating the coating composition and/or the liquid material, according to the present invention, coating may be performed by using an inkjet printer head while patterning is performed. According to the method described above, since amounts of starting materials to be used can be minimized, it is particularly effective to perform cost reduction.